In recent years, attention has been paid to optical image measurement technologies that form images of the surface and inner sections of measured objects using light. Optical image measurement technologies are not invasive to the human body, unlike radiographic imaging, making them promising particularly in the medical field. Among these technologies, significant progress is being achieved in the fields of ophthalmology, dentistry, dermatology, etc. Moreover, technologies are also being applied in the field of biology and engineering.
Optical coherence tomography (OCT) is a representative method of optical image measurement technologies. With this method, it is possible to measure at high resolution and highly sensitivity due to the use of interferometers. Moreover, because weak wideband light is used for illumination, this method is advantageous in that it provides high safety to the human body, etc.
Examples of apparatuses using OCT (OCT apparatus) include the apparatus described in Patent Document 1. This OCT apparatus generates interference light by superimposing light passing through the cornea (signal light) and light passing through a reference object (reference light) to form an image of the cornea, based on the detection results of this interference light. Accordingly, the obtained image is an image of a cross-section that is substantially perpendicular to the propagating direction of signal light. This method is referred to as a full-field type or an en-face type. This type of OCT apparatus is characterized by being able to obtain high-powered and high resolution images, compared to other types, and for example, it can be applied to observe microstructures (cells, etc.) of the cornea.
Other types of OCT apparatuses include swept source OCT, Fourier domain OCT, polarization-sensitive OCT, Doppler OCT, etc.
When measuring moving measured objects such as a living eye with OCT apparatuses, a phenomenon, namely fringe washout, may occur (for example, refer to Patent Document 2). Fringe washout is a phenomenon in which the detection sensitivity of the interference light decreases as a result of the effect of movement of measured objects (that is, the interference fringes become unclear), causing image definition to decrease.
An explanation is provided regarding fringe washout which occurs with full-field type OCT apparatuses. When measured objects move in the optical axial direction of the signal light, a Doppler frequency shift occurs with the signal light. Interfering components Iinterference of interference signals generated as the signal light interferes with the reference light that is reflected on a reference mirror (rest state) are expressed in the following formula in which amplitude is modulated.[Formula 1]Iinterference=√{square root over (IsIr)} sin(2πfDopplt+φ)  (1)
Here, if the optical refraction index of a measured object is n, the speed of the measured object in the optical axial direction is v, and the wavelength of the signal light is λ, the amount of Doppler frequency shift (Doppler frequency shift amount) is expressed as fDoppl=2 nv/λ. Moreover, in Formula (1), Is is the intensity of the signal light, Ir is the intensity of the reference light, and φ is the initial phase difference.
When the interfering components in Formula (1) are detected with an electric charge storage-type light detecting device such as a CCD, these interfering components are integrated within a storage time (may also be referred to as exposure time) of the device and can be represented by the following formula. Note that < > is a integration sign.[Formula 2]Iinterference=√{square root over (IsIr)} sin(2πfDopplt+φ)  (2)
As is clear from Formula (2), the term expressed in the sine function resulting from the integration effect of the device is averaged. This is the fringe washout phenomenon. This type of decrease in the detection sensitivity attributable to movement of measured objects is described in, for example, Non-Patent Document 1. This document describes that the larger the movement velocity of the signal light in the optical axial direction is, the weaker the interference signals are (that is, the detection sensitivity decreases).